The effectiveness of the immune system declines with age, leaving the elderly susceptible to life-threatening diseases. This dysregulation of the aged immune system results in poor humoral responses to T-dependent, protein antigens. Antibodies produced by aged individuals show decreased isotype switching, fewer somatic mutations, and increased autoreactivity. This predominance of "natural immunity" is accompanied by an increase in the Bl cell population. However, not all humoral responses are affected; while TD responses appear to decline, T-independent responses appear to increase. Here, we approach the issue of aging from a novel perspective - a homogeneous population of antigen-binding cells by using VDJ knock-in B cells (QM B cells). These transgenic cells retain the ability to undergo normal events associated with immunopoiesis such as isotype switching and affinity maturation. Additionally, since the cognate antigen, nitrophenyl (NP), is not found in the environment, this provides us with a unique opportunity to examine the effects of antigen exposure on the skewing of B cell subsets during aging. Our previous data has suggested that aged QM B cells do retain the ability to undergo antigen-induced expansion, acquisition of germinal center markers and production of high affinity, isotype switched antibodies. The specific aims of this proposal are 1) Determine the nature of T cell help preferred by aged QM B cells. Do the cells respond better to Thl help than Th2 help? 2) Determine if B cell subsets are skewed in aged naive QM animals vs QM animals exposed periodically to antigen, 3) Determine if the aged Bl cells are functional and able to respond to TI and TD antigens. To accomplish Aim 1, adoptive transfers will be performed with in vitro polarized Th cells. To accomplish Aim 2, QM mice will be left na[unreadable]ve or immunized periodically with NP-OVA. At various ages, subsets will be analyzed by flow cytometry to determine the populations of Bla, Bib and B2 cells. Aim 3 will be accomplished by adoptive transfer of aged Bl cells into scid recipients followed by immunization with both TI and TD antigens. Responses will be measured by allotype-specific ELISA. [unreadable] [unreadable]